This time it’s really gone. Previous claims have been made, but NASA says that after a 35-year journey Voyager 1 has indeed reached interstellar space. And here’s the evidence:

[SOUND]

I’ll tell you what that sound is in a moment.

It appears that the spacecraft left the sun’s heliosphere more than a year ago. In August 2012, Voyager registered a drop in charged particles emanating from the sun. At the same time it was hit with higher levels of interstellar cosmic rays. But in the absence of corroborating evidence from plasma or magnetic field measurements, Voyager scientists were not convinced that Voyager 1 had truly departed.

But in March 2012, the sun belched a big batch of plasma. The reverberation from that eruption took a year to reach Voyager. From the vibration of plasma in Voyager’s vicinity, scientists were able to measure the density of the plasma. That’s what you heard earlier: Voyager’s record of seven months of plasma vibrations condensed to a few seconds. [Donald Gurnett et al., In Situ Observations of Interstellar Plasma with Voyager 1, in Science]

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